This invention pertains to voltage pulse generators, and more particularly, is concerned with spiral line pulse generators.
Spiral line pulse generators are described in U.S. Pat. No. 3,289,015 and in "Novel Principle of Transient High Voltage Generator" by Fitch et al, Proc-IEEE, Vol. 111, No. 4, April 1964.
A conventional spiral line pulse generator 10 is shown schematically in FIG. 1. A pair of conductors 11 and 12 in the form of elongated strips of conductive material are rolled together to form a double arm, multiple turn spiral structure. FIG. 2 is a partial cross-sectional view of the spiral line pulse generator 10 illustrating the layered construction of the device. A four layered arrangement of alternating conductors and insulators, including the conductive strips 11 and 12 and a pair of insulative strips 13 and 14, are rolled on a form 15 in a multiple turn spiral configuration. Form 15 provides mechanical rigidity. The conductive strips 11 and 12 are separated by the dielectric material of insulating strips 13 and 14 at every point in the spiral configuration. Conductive strip 12 runs from point 16 to point 17 while conductive strip 11 runs from point 18 to point 19. In the present example, a switch 20 is coupled between conductive strips 11 and 12 at or near the points 16 and 18; however, switch 20 can be across points 17 and 19 or even across the center of the strips. A voltage V.sub.o is applied between the conductive strips 11 and 12. Prior to the closing of the switch 20, the conductive strip 12 has a uniform potential between the points 16 and 17, and the conductive strip 11 has a uniform potential between the points 18 and 19. The voltage difference between the innermost and the outermost turns of the spiral configuration is at most V.sub.o. This can be seen by summing the electric field vectors at time t=0 as shown in FIG. 2. When switch 20 is rapidly closed, a field reversing wave propagates along the transmission line formed by the conductive strips 11 and 12. When the wave reaches the points 17 and 19, at time t=.tau., the potential difference between the points 17 and 19 is nV.sub.o, where n is the number of turns in the spiral configuration, due to the absence of cancelling static field vectors. The propagating wave undergoes an in-phase reflection at the points 19 and 17 when these points are terminated in a high impedance or are open circuited. This results in an additional increase in the potential difference between the innermost and outermost conductors with a maximum occurring at time t=2.tau., at which time the field vectors are aligned as shown in FIG. 1. The output taken between points 17 or 19 and point 16 reaches a maximum voltage of 2nV.sub.o at t=2.tau. after the closure of the switch 20.
Proud et al in U.S. Pat. No. 4,325,004 teaches the use of a spiral line pulse generator to ignite a discharge lamp. For this, as well as other uses, cost and size are important factors. In particular, the dielectric material used for the insulating strips in high temperature applications, such as lamps, is expensive. In lamp circuits it is desirable to locate the pulse generator in the base of the lamp. Both size and cost can be reduced if a new spiral line generator could be devised which yields the voltage output of conventional spiral line pulse generators, but with fewer turns.
Furthermore, it has been observed that the output of a conventional spiral line pulse generator drops considerably when contained in a metal housing such as a lamp base. This loss is due to the shorted turn effect of electromagnetic fields extending to the metal housing. It would be desirable to reduce this loss due to the shorted turn effect by concentrating the electromagnetic field close to active layers of the spiral line structure.